FRONT brake disc rotor & wheel I.D. needed (non-XS650)

[DogBunny]

Acquired in a trade. Anyone recognize this? I think the give-away will be this brake rotor. The folding lock-tabs appear to be original and un-molested. The rim is chrome-steel and is labeled TAKASAGO 18 X 1.85. The axle diameter is only 15mm (smaller than an XS650 front axle). Someone put a chain sprocket on it, but this is a front wheel.

 

[xjwmx]

lots of disks on ebay for gs450 have that Y business. sprocket is a match too from what i can tell

https://www.ebay.com/itm/Suzuki-GS4...Suzuki&hash=item5d7d8bcdbd:g:RBMAAOSw4ztdATdi

 

[DogBunny]

Thanks for the guidance, it helps, but this is turning out to be a real head-scratcher.
I now think that this is a front wheel that someone put a chain sprocket on. The GS450 does use the "Y" pattern brake rotor, but the GS450 uses a 19" front wheel, and this is an 18" front wheel. (The GS450 uses a drum rear brake). I think this came off of something smaller than a 450, like a GS250, which does have an 18" front wheel.
I'm still trying to zero in on this one, and still taking suggestions.

 

[DogBunny]

Just noticed that the hub is slotted for speedometer drive gear, so this is definitely a front wheel.

 

[DogBunny]

Okay I'm pretty sure that this is a front wheel for a 1980-1981 Suzuki GN400.
Thanks again for your help.